I'll start by telling you what my system consists of. I have an HQST Mppt 20 amp 12/24 charge controller hooked to one Longi 310 watt panel (33.3 V, 9.3A). This seems to be working fine. I also have an HQST Mppt 40amp 12/24V charge controller. I tried hooking it to two LG 350 watt panels (35.3V, 9.92A) connected in series. Smoke from the 40A charge controller. My inverter is a Renogy 12V 2,000 watt. How should this be wired?
I need help with my small off grid set-up
- Thread starter jdctoo
- Start date
mikefitz
Solar Wizard
- Joined
- May 28, 2020
- Messages
- 2,979
Was this connected to a battery before the panels were connected?
JBertok
Network Engineer
When equipping an MPPT with panels, there are voltage, current (amperage), and wattage specifications that you have to not to exceed.
From the panels collect ratings of:
- Open Circuit Voltage (Voc)
- Short Circuit Current (Isc)
- Maximum Power (Pmax)
Your panels were in series, and you've quoted the Rated Power Voltage (Vm) or (Vmp) of 35.3V, but the Open Circuit Voltage (Voc) needs to be considered when calculating limits. I pulled an LG 350W NeON 2 spec to look at and find a Voc of 41.3V. Similarly, the current quoted at 9.92A is the MPP Current (Impp) but the Short Circuit Current (Isc) should be considered instead when calculating current limits for fuses and wiring. Your MPPT doesn't list an input current limit in the specs but it can be extrapolated from the voltage and wattage input limits.
So, for the 2-panel array:
(Voc) = 41.3V * 2 panels in series = 82.6V
(Pmax) = 350W * 2 panels = 700W
For the HSQT 40A MPPT:
- Maximum PV Input Voltage = 100V, > 82.6V = SAFE
- Maximum PV Input Power = 600W (12V mode) < 700W = EXCEEDED
The reason this happened is that the quoted amperage rating of an MPPT in its listing refers to the battery-side output maximum, not the PV side input. For this unit you have, it's 40 Amps output, whether set to 12 Volts or 24 Volts. Most of the cheaper MPPTs do not have much ability to over-provision the array input, if at all; this is a specific feature that has to be accommodated by the design of the MPPT circuitry. Yours actually has a little bit since it's rated at 600W input even though its output maximum will be 480W@12V and 536W@13.4V and 564W@14.1V, etc. But, 700W input is too much, per spec.
700 Watts divided by 12 Volts equals 58 Amps. Even at the maximum allowable voltage to the batteries (14.8V) you're still looking at 47.3A which is too much. Panels don't put out their full rating, and there is some loss in the MPPT, but it's still way too much current for this model MPPT. Those 2 panels together will require the 60A model, at minimum, to be safe. Hope this helps.
From the panels collect ratings of:
- Open Circuit Voltage (Voc)
- Short Circuit Current (Isc)
- Maximum Power (Pmax)
Your panels were in series, and you've quoted the Rated Power Voltage (Vm) or (Vmp) of 35.3V, but the Open Circuit Voltage (Voc) needs to be considered when calculating limits. I pulled an LG 350W NeON 2 spec to look at and find a Voc of 41.3V. Similarly, the current quoted at 9.92A is the MPP Current (Impp) but the Short Circuit Current (Isc) should be considered instead when calculating current limits for fuses and wiring. Your MPPT doesn't list an input current limit in the specs but it can be extrapolated from the voltage and wattage input limits.
So, for the 2-panel array:
(Voc) = 41.3V * 2 panels in series = 82.6V
(Pmax) = 350W * 2 panels = 700W
For the HSQT 40A MPPT:
- Maximum PV Input Voltage = 100V, > 82.6V = SAFE
- Maximum PV Input Power = 600W (12V mode) < 700W = EXCEEDED
The reason this happened is that the quoted amperage rating of an MPPT in its listing refers to the battery-side output maximum, not the PV side input. For this unit you have, it's 40 Amps output, whether set to 12 Volts or 24 Volts. Most of the cheaper MPPTs do not have much ability to over-provision the array input, if at all; this is a specific feature that has to be accommodated by the design of the MPPT circuitry. Yours actually has a little bit since it's rated at 600W input even though its output maximum will be 480W@12V and 536W@13.4V and 564W@14.1V, etc. But, 700W input is too much, per spec.
700 Watts divided by 12 Volts equals 58 Amps. Even at the maximum allowable voltage to the batteries (14.8V) you're still looking at 47.3A which is too much. Panels don't put out their full rating, and there is some loss in the MPPT, but it's still way too much current for this model MPPT. Those 2 panels together will require the 60A model, at minimum, to be safe. Hope this helps.
